This type of zoom lens is disclosed, e.g., in JP 2001-311872 A, JP 2000-98222 A, JP 2001-51195 A, and JP 2002-148515 A.
In a projector that uses a transmission-type spatial optical modulating element for three primary colors of red, green and blue, a prism (color composition prism) for composing the three colors is located between a projection lens and a spatial optical modulating element. Because of this, the projection lens requires a long back focus. Since spectral characteristics of the color composition prism are dependent on the incident angle, the projector requires an optical system that allows the position of the pupil on the shorter side conjugate distance to be sufficiently far away from the spatial optical modulating element, i.e., an optical system having a telecentricity.
There has been an attempt to improve the degree of freedom in installation of a projector by deviating the optical axis of a projection lens from the center of a projected image. A projector that can vary the amount of deviation between the optical axis of the projection lens and the center of the projected image also has been proposed.
In a projector that uses a reflection-type spatial optical modulating element, a light source should be placed on the same side of the spatial optical modulating element as a projection lens. In this case, it is necessary to arrange the light source so that the illumination light from the light source does not interfere with the projection lens. For this purpose, e.g., the optical axis of the projection lens deviates from the center of the spatial optical modulating element. In this method, however, the amount of deviation between the optical axis of the projection lens and the center of the spatial optical modulating element cannot be varied and has to be fixed.
To vary the amount of deviation between the optical axis of the projection lens and the center of the projected image, it has been proposed that a prism utilizing total internal reflection is located between the projection lens and the spatial optical modulating element. In a projector using this total reflection prism, the projection lens requires a long back focus for arranging the total reflection prism and a telecentricity for maintaining the total reflection conditions. These requirements are the same as those for the projection lens of the projector including a transmission-type spatial optical modulating element.
When a projector includes a reflection-type spatial optical modulating element, a total reflection prism, and a telecentric-type projection lens, unnecessary light is generated and becomes a serious problem. Such unnecessary light is not generated in a projector that includes a transmission-type spatial optical modulating element, a color composition prism, and a telecentric-type projection lens.
The reflection-type spatial optical modulating element has a high reflectance for light from the projection lens. Therefore, unnecessary light caused by the projection lens is reflected from the reflection-type spatial optical modulating element, passes through the projection lens again, and is projected onto the screen.
For the projector including the transmission-type spatial optical modulating element, the color composition prism, and the telecentric-type projection lens, unnecessary light does not become a serious problem because the reflectance of the transmission-type spatial optical modulating element is low. While the transmission-type spatial optical modulating element has a reflectance of 4%, the reflection-type spatial optical modulating element has a reflectance of 90%, which is 22.5 times as high as the reflectance of the transmission-type spatial optical modulating element.
In a projector in which the amount of deviation between the reflection-type spatial optical modulating element and the optical axis of the projection lens is fixed, the amount of deviation is large, and thus the reflection-type spatial optical modulating element is not located at a symmetrical position with respect to the optical axis of the projection lens. Generally, unnecessary light often is generated at the symmetrical position with respect to the optical axis of the projection lens. Therefore, in the projector in which the amount of deviation between the reflection-type spatial optical modulating element and the optical axis of the projection lens is fixed, the unnecessary light caused by the projection lens is not likely to be reflected from the reflection-type spatial optical modulating element, pass through the projection lens again, and is projected onto the screen.